Choosing a planted tank LED without a PAR meter
Lighting threads — and Aquarium Co-Op's unusually good light guides — assume a PAR meter, a chemistry set, and a strong opinion about spectrum. I own none of those things. Between four fixtures across three tanks — two Chihiros, one Twinstar, one budget Nicrew that outlived its reputation — the useful lessons were all about behavior, not specifications.
Dimming is the real feature
The fixture's job is to be too strong and then be turned down. Every fixture on the market now over-lights a shallow planted tank at 100%; the difference between a good purchase and a bad one is whether the dimming is stepped, app-controlled, and remembers its setting after a power cut. The Nicrew died not as a light but as a controller — it forgot its intensity on every outage and cooked the tank twice before I replaced it.
Height beats watts
The same fixture at 20 cm above the waterline and at 35 cm are two different tanks. Mounting height is the free dimmer: high fixture, low percentage, even spread. My 60P runs its A-II at 60% because it sits on the factory legs over open water — the numbers from the 60P log are inseparable from that geometry.
Spectrum: ignore the rhetoric
"Full spectrum for plants" and "RGB for colors" describe overlapping things. Plants use red and blue efficiently; the eye calls the result of that mix "pink." Pink fixtures make red plants redder and green plants flatter; white-heavy fixtures do the reverse. Pick by which tank you want to photograph, not by plant health — the health difference between the two camps, at matched intensity, has been invisible here.
The two-week window
A new light's setting is decided in the first two weeks, and the tank tells you how. Green spot algae on the glass means too much, too steady — drop intensity or shorten the photoperiod before scraping. Diatoms on the substrate mean something else entirely and usually resolve on their own. Stretched, pale stems mean too little — raise intensity before lengthening hours. Adjust once, wait two weeks, adjust again. Every algae war I have lost began with three adjustments in one week.
On the DSM side
Dry starts want less, not more: emersed seedlings scorch under fixtures that submerged plants shrug off. The 40% rule from the dry-start post-mortems is the same fixture at a lower seat of the same stand.
Buy any fixture with honest stepped dimming and a memory. Then spend the money you saved on a timer. The timer is the actual light.